Trump on Friday also used the London train attack to renew calls for his controversial travel ban on travelers from several predominantly Muslim countries, which is the subject of fierce legal battles in the USA but has partially gone into effect.

And a police spokesperson told CNN that Trump's comment is "pure speculation given we don't know who involved".

At least 22 people were injured in London Friday morning when what police are describing as a crude device was exploded on a crowded commuter train.

Prime Minister Theresa May put Britain on the highest security level of "critical" late on Friday, meaning another attack may be imminent, and deployed soldiers and armed police to strategic locations such as nuclear power plants.

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Police said they were treating the explosion as a "terrorist incident" and have launched an "urgent manhunt" for the suspect.

"I think he means generally that this kind of activity is what we are trying to prevent", McMaster said.

This isn't the first time Trump has used terrorist attacks in London to call for his travel ban.

President Donald Trump speaks to members of the media before boarding Marine One on the South Lawn of the White House in Washington, Thursday, Sept. 14, 2017, for a short trip to Andrews Air Force Base and then on to Fort Myers, Fla.to meet with citizens impacted by Hurricane Irma. Western intelligence officials have questioned similar claims in the past, saying that while Islamic State's jihadist ideology may have inspired some attackers, there is scant evidence that it has orchestrated attacks. "We have to be tougher and we have to be smarter". Along with advocating for a "larger, tougher" travel ban, the President also suggested shutting down the internet.

London Mayor Sadiq Khan said in a statement that his city will not be defeated by terrorists.

Meanwhile, The United States Supreme Court is expected to hear the oral argument to decide on the travel ban's constitutionality.

Trump is using the incident to defend his travel ban against six Muslim-majority countries.